This invention relates to methods and apparatus for measuring the spectral energy in the spectrum of a signal, such as a radio-frequency (r.f.), intermediate frequency (i.f.) or baseband signal, in particular in the case where the signal contains intervals of random modulation to be included in the measurement and an interval of non-random modulation to be omitted from the measurement.
The rapidly increasing adoption of mobile communications facilities, such as cellular telephone systems, has led to a huge and growing demand for the associated user devices, such as mobile telephone handsets and other mobile stations (MS). Because these devices are typically radio transmitters as well as receivers, it is essential that each device is fully tested by the manufacturer before shipment, to ensure that no undesirable radio emissions are generated which might cause interference to other users. Equally however, the manufacturers desire that these tests should be accomplished as quickly as possible, to minimise delays they introduce into production timescales.
In the case of the GSM system, for example, the relevant European Telecommunication Standards (GSM 05.05, 11.10, 11.20 and 11.21) provide for measurements of a variety of parameters, including the Output RF Spectrum (ORFS). Two ORFS measurements are defined, to ascertain the distribution of energy in the spectrum of the output RF signal of a GSM transmitter (mobile or fixed). The first measures effects of the modulation process and the second measures effects of switching transients. A GSM signal is normally burst modulated and this causes the switching transients. The measurement of ORFS due to modulation is intrinsically noisy in character, because the measurement bandwidth is defined to be significantly less than the modulation bandwidth, so a measurement of a single signal burst has a very low statistical significance. Therefore measurements of multiple bursts must be averaged to provide a useful result.
Each burst incorporates a xe2x80x9cmidamblexe2x80x9d which is modulated with a fixed data pattern to assist receiver operation. Inclusion of this midamble in the measurement of ORFS due to modulation would conflict with the underlying assumptions involved in averaging multiple measurements, so it is necessary to exclude the fixed or non-random midamble modulation from this measurement.
For these reasons the GSM standards require that, at a given frequency in the spectrum, an energy measurement is made of the interval of a burst extending from a point occurring after 50% of the burst has elapsed, and excluding the midamble, until 90% of the burst has elapsed; in practice this involves selecting at least forty bits in the range from bit 87 to bit 132 inclusive. This measurement is typically repeated for 200 such bursts and the resulting values are averaged. This process is repeated at multiple frequency offsets to complete the determination of ORFS due to modulation.
The measurement of 200 bursts at one frequency setting cannot be completed any faster than the bursts actually occur, irrespective of the speed of the measurement equipment. The GSM frame rate is 216 bursts per second, so a 200-burst measurement at one frequency unavoidably takes the best part of one second. The duration of this determination is a significant contribution to the overall duration of the measurements required for complete testing of each MS during manufacture.
It is an object of this invention to reduce the duration of the measurement of the ORFS due to modulation while maintaining the statistical significance of the measurement results.
According to one aspect of this invention there is provided a method of measuring the spectral energy within a measurement bandwidth in the spectrum of a modulated signal which has a modulation bandwidth exceeding said measurement bandwidth and which contains at least two intervals of random modulation to be included in the measurement and an interval of non-random modulation to be omitted from the measurement, comprising the steps of:
receiving a signal containing the intervals of random and non-random modulation;
identifying the interval of non-random modulation in the signal;
identifying a first interval of random modulation in the signal in predetermined relationship with the interval of non-random modulation;
identifying a second interval of random modulation in the signal in predetermined relationship with the interval of non-random modulation;
selectively measuring the energy content of the first and second intervals of random modulation, while excluding the interval of non-random modulation; and
averaging the results of those selective measurements.
According to another aspect of this invention there is provided apparatus for measuring the spectral energy within a measurement bandwidth in the spectrum of a modulated signal which has a modulation bandwidth exceeding said measurement bandwidth and which contains at least two intervals of random modulation to be included in the measurement and an interval of non-random modulation to be omitted from the measurement, comprising:
a receiver for receiving a signal containing the intervals of random and non-random modulation;
an identifier for identifying:
the interval of non-random modulation in the signal;
a first interval of random modulation in the signal in predetermined relationship with the interval of non-random modulation; and
a second interval of random modulation in the signal in predetermined relationship with the interval of non-random modulation;
a monitor for selectively measuring the energy content of the first and second intervals of random modulation, while excluding the interval of non-random modulation; and
an averager for averaging the results of those selective measurements.